Loading device for measuring stiffness of structural member over time, monitoring system, and method thereof

ABSTRACT

A loading device, a monitoring system, and a method thereof can measure stiffness of a structural member (SM), such as a bone, and monitor progress or property over time. The loading device includes two types of displacement sensors, one type being an antenna. As the SM, which is in a magnetic or electromagnetic field and electromagnetically coupled to the antenna without contact, undergoes displacement under known loads, characteristics of the electromagnetic field coupling between the antenna and the SM change over time due to the displacement of the SM. The shift in the characteristics of the electromagnetic field coupling between the antenna and the SM can be used to determine the displacement of the SM. Based on the changes in the displacement over time, diagnosis of the SM being monitored over an evaluation period can be made.

BACKGROUND

The U.S. Patent Application Publication No. 2019/0162606, correspondingto published PCT application WO2018/195437 (hereafter Reference 1)discloses that an in-dwelling strain sensor is not needed to monitorstrain applied to a structural member. Instead, it discloses using anantenna spaced from a structural member without the antenna makingcontact therewith.

There still remains a need for a methodology and a device/system forapplying a bending load to a structural member, which can be a person'sextremity for example, and measuring the deflection of the extremity dueto the applied load to accurately determine the stiffness thereof overtime. In normal fracture healing, stiffness of the bone-hardwareconstruct increases over time due to calcification of the fracturedcallus. It would be desirable to track these changes over time tomonitor the progress and predict cases of delayed unions or non-union,allowing medical professionals to take appropriate action.

The present invention addresses these needs.

SUMMARY

The present methodology and device/system can monitor a structuralmember (SM), for example, healing of fractured bone fixed with implantedorthopedic hardware, using two types of sensors, one of which is anon-contact type.

One aspect of the present invention is a loading device for measuringstiffness of SM. The loading device can include a rigid frame assembly,a driving mechanism, a first sensor, a plurality of second sensors, anda third sensor.

The frame assembly can include a base, distal frame, a movable support,a plurality of first loading member, and at least one second loadingmember. The base is restable on a support. The distal frame can be heldspaced from the base at a predetermined distance. The movable support ismovable between the base and the distal frame. The first loading memberscan be disposed spaced along a length of the distal frame and configuredto support the SM. The at least one second loading member can bedisposed on the movable support and configured to support the SM.

The first loading members and the at least one second loading member canbe disposed facing each other. The first loading members and the atleast one second loading member can provide a three-point bending orfour-point bending configuration. The first loading members can includetwo first loading members and the at least second loading member caninclude one second loading member to provide the three-point bendingconfiguration.

The driving mechanism can move the movable frame to drive the movablesupport and apply a load to the SM and create multiple bending pointswith respect to the first loading members and the at least one secondloading member. The driving mechanism can move the at least one secondloading member toward and away from the first loading members to applyan opposing force to the SM from opposing sides.

The first sensor, which can be a load sensor, measures a load applied bythe driving mechanism to the movable support. The first sensor can bedisposed between the driving mechanism and the movable frame.

The second sensors, which can be displacement sensors, measure a firstdeflection of the SM undergoing loading. Each of the second sensors canbe adjustably positionable along the length the distal frame. The secondsensors can include three displacement sensors positioned along thelength of the distal frame. Two of the three displacement sensors can bedisposed adjacent to the two first loading members, and the thirddisplacement sensors can be located at the midpoint between the twofirst loading members.

The third sensor can measure a second deflection of the SM without usingany strain sensing device attached to the SM or contacting the SM.Specifically, the third sensor comprises an antenna disposed spaced fromthe SM, secured to the distal frame, and is configured to (a) induce,using a first electrical signal, a magnetic or electromagnetic field inthe vicinity of the SM to create a coupling of the magnetic orelectromagnetic field between the antenna and the SM, wherecharacteristics of the magnetic or electromagnetic field couplingbetween the antenna and the SM are associated with a distance betweenthe SM and the antenna, and (b) output a second electrical signalrepresenting the magnetic or electromagnetic field coupling between theantenna and the SM, without using any strain sensing device directlyattached to the SM.

The antenna can comprise a first coaxial cable including a first end anda second end. A mount can be secured to the distal frame and hold theantenna in place. Specifically, the mount is configured to maintain thefirst coaxial cable stationary, in relation to the distal frame, in acoil configuration between the first and second ends. The mount caninclude a pair of spaced plates secured to the distal frame. Each of thepair of spaced plates can include a plurality of spaced through holes,through which the first coaxial cable is kept in the coil configuration.

The antenna can be a dipole antenna that provides a S11 parameter andfunctions as a displacement sensor, which is of a different type fromany of the second sensors. In this respect, the antenna can furtherinclude a second coaxial cable including a third end and a fourth end.The spaced through holes also keep the second coaxial cable in the coilconfiguration between the third and fourth ends.

The first end of the first coaxial cable can include a connector forconnecting to a signal processor, such as a network analyzer, so thatonly the signal from the first coaxial cable is used to obtain thesecond deflection curve.

The first coaxial cable includes a first shield and a first centerconductor, and the second coaxial cable includes a second shield and asecond center conductor. The first and second shields can beelectrically connected. The first center conductor at the second end andthe second center conductor at the third and fourth ends can terminatewithout any connection. A predetermined length of the first centerconductor can be exposed at the second end and the predetermined lengthof the second center conductor can be exposed at the fourth end. Theexposed first and second conductors can be maintained substantiallyparallel to each other.

The stiffness of the SM can be determined from (a) a first slope of afirst deflection curve obtained from the plurality of second sensorsversus the applied load curve obtained from the first sensor and (b) asecond slope of a second deflection curve obtained from the third sensorversus the applied load curve.

Another aspect of the present invention is a monitoring system that canmeasure and monitor stiffness of the SM over an evaluation period. Themonitoring system can include the loading device, which uses the loadsensor, the displacement sensors, and the antenna, a controller, and ahardware interface.

The hardware interface is configured to receive the second electricalsignal from the antenna, a third electrical signal from the load sensor,and a fourth electrical signal from each of the displacement sensors,and convert each of the received second, third, and fourth signals thatare readable by the controller.

The controller includes a memory storing instructions and a processorconfigured to implement instructions stored in the memory and execute aplurality of tasks, including a first determining task, a repeatingtask, a second determining task, and third determining task.

The first determining task receives the converted second electricalsignal from the hardware interface and determines characteristics of themagnetic or electromagnetic field coupling between the antenna and theSM.

The repeating task repeats the first determining task to obtain aplurality of characteristics of the magnetic or electromagnetic fieldcoupling between the antenna and the SM at a predetermined interval overthe evaluation period.

The second determining task determines a shift in characteristics of themagnetic or electromagnetic field coupling between the antenna and theSM after each occurrence of the first determining task determining thecharacteristics of the magnetic or electromagnetic field couplingbetween the antenna and the SM at the predetermined interval, orcollectively at the end of the evaluation period.

The third determining task determines a temporal change in relativedisplacement of the SM and determine the stiffness of the SM over theevaluation period, based on the first slope and the second slope.

The hardware interface can be a network analyzer configured to (a)output the first electrical signal to the antenna to induce the magneticor electromagnetic field, (b) receive the second electrical signal fromthe antenna, and (c) also determine the characteristics of the magneticor electromagnetic field coupling between the antenna and the SM basedon the received second electrical signal.

Another aspect is a method of monitoring the stiffness in the SM as theSM undergoes a displacement under a load. The method includes (a)disposing the antenna spaced from the SM so that the antenna does notcontact the SM, (b) inducing the magnetic or electromagnetic field inthe vicinity of the SM and creating the coupling of the magnetic orelectromagnetic field between the antenna and the SM, (c) applying aload to the SM to create multiple bending points, (d) monitoring eachload applied to the SM using the load sensor and storing each load datain a storage device, (d) measuring the amount of deflection the SMundergoes for each applied load using the displacement sensors, eachdisposed adjacent to one of the multiple bending points, and storingdeflection data in the storage device, (e) obtaining the secondelectrical signal from the antenna each time a load is applied to the SMand determining a distance between the antenna and the SM for eachapplied load and storing determined distance information in the storagedevice, (f) determining a first slope of a first deflection curveobtained from the stored deflection data versus an applied load curveobtained from the stored applied load data, (g) determining a secondslope of a second deflection curve obtained from the stored determinedcharacteristics versus the applied load curve, and (h) determining thestiffness of the SM based on the first slope and the second slope.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates an overall side perspective view of a loading deviceaccording to the present invention, with a structural member shown inphantom.

FIG. 2 illustrates a front view of the loading device.

FIG. 3 illustrates a top view of a distal frame of the loading device.

FIG. 4 illustrates an enlarged view of an antenna configuration moreclearly illustrating the arrangement of the exposed center conductor ofthe coaxial cables.

FIG. 5 illustrates a bottom view of the distal frame.

FIG. 6 illustrates an alternative embodiment of a movable support thatincludes multiple lower loading members to provided multiple lowerloading points.

FIG. 7 schematically illustrates an embodiment of a system that canmonitor changes in the structural member.

FIG. 8 illustrates a controller or computer that can determine theinstantaneous or temporal change in stiffness of the structural member(over an evaluation period).

FIG. 9 illustrates an operational diagram, namely a flowchart of thepresent system for monitoring the stiffness in the structural memberbased on one sensor type (antenna).

FIG. 10 illustrates another operational diagram, namely a flowchart ofthe present system for monitoring the stiffness in the structural memberbased on another sensor type (displacement sensors).

FIG. 11 illustrates an example curve showing antenna resonant frequencyversus the applied load.

FIG. 12 illustrates slopes (mean and standard deviation) of the a linearsensor displacement versus the load curves for three compliance groups.

FIG. 13 illustrates sloped (mean and standard deviation) of thecorrected displacement versus the load curves for the different loadingpoint distances and for intramedullary rods representing an unhealedfracture.

FIG. 14 illustrates radiographs of the eight testing conditions of asheep tibia showing progressively destabilizing osteotomies.

FIGS. 15A-15B illustrate slopes (mean and standard deviation) of the (A)antenna resonant frequency versus load curves, and (B) corrected linearsensor displacement versus the load curves.

DETAILED DESCRIPTION

The present loading device and monitoring system can measure thestiffness of a structural member (SM) that deflects when a force isapplied.

Referring to FIG. 1, the loading device LD includes a loading frameassembly FA, a driving mechanism DM, a (first) sensor S1, a plurality of(second) sensors S2, and a (third) sensor 3. The frame assembly FA canrest on a stable surface SS, such as a sturdy table or a floor, andincludes upper and lower loading members LMu, LMl that act as loadingpoints on two sides of the SM, such as a fractured tibia fixed with anintramedullary nail or other structure support, in multiple-pointbending configuration, such as a three-point bending (shown in FIG. 1)or four-point bending configuration (shown in FIG. 6). The locations ofthe loading points can be adjustable to accommodate various lengths ofthe SM. The loading members, including multiple upper (first) loadingmembers LMu, that are positioned spaced from each other at a desireddistance and rigidly mounted to the frame assembly FA, and at least onelower (second) loading member LMl, which is driven toward or away (seethe double arrows in FIG. 1) from the upper loading members LMu usingthe driving mechanism DM secured to the frame assembly FA.

The driving mechanism DM can be a linear screw jack, which can bemanually controlled by a hand crank DM3 connected to a rotating shaftDM2. The sensor S1, which can be a load sensor, such as a load cell, canbe connected to a movable support or bed FA3 to measure the forceapplied by the driving mechanism DM.

The loading device LD in the present development uses two differenttypes of sensors, namely a first type that makes a physical contact withthe SM and/or material surrounding the SM, such as skin and tissue, anda second type that makes no physical contact with the SM or the materialsurrounding the SM. The stiffness of the SM can be determined from a(first) slope of a first deflection curve obtained from the plurality ofsensors S2, which is one of the two sensor types, versus an applied loadcurve, based on the driving mechanism DM applying different loads, whichis obtained from the sensor S1, and a (second) slope of a seconddeflection curve obtained from the sensor S3, which is the other of thetwo sensor types, versus the applied load curve. The second sensors S2measure the first deflection of the SM and the third sensor S3 measuresthe second deflection of the SM. Combining both types of sensors allowsmultiple measurements of stiffness, providing for a more robust andreliable measurement system.

The frame assembly FA provides a rigid platform on which the SM canrests, and has sufficient rigidity so that any deformation it mayundergo would be negligible versus the deflection that the SM undergoeswhen it is stressed. Specifically, the frame assembly FA includes a baseFA1 that is restable on a stable support SS, such as a floor or a table,a distal frame FA2 that is held spaced from the base FA1 at apredetermined distance, which can be adjustable, and a movable supportFA3 that is configured to be movable between the base FA1 and the distalframe FA2.

Referring to FIGS. 1-6, the base FA1 and the distal frame FA2 can bemade of steel, aluminum, rigid plastic, or carbon fiber material, or anycombination thereof. Specifically, in the illustrated embodiments, thedistal frame FA2 can be constructed from a pair of rigid main beams FA2a that are spaced apart parallel along a horizontal plane and secured inthat position using a plurality of support plates FA2 b and cross beamsFA2 c so that the main beams and the cross beams form a rigid structurethat is not prone to deflection when the SM undergoes stress. The mainand cross beams also can be stacked to provide an even more rigidplatform. The beams can include channels/slots for anchoring bolts andnuts, such as shown in FIGS. 1-3 and 5 for attaching accessories, suchas the second sensors 2 and the loading members LMu, LMl. The main andcross beams are commercially available. For example, heavy dutycommercially available T-Slot aluminum extrusion rails can be used, suchas available from McMaster-Carr(https://www.mcmaster.com/t-slotted-framing) or 80/20 Inc(https://8020.net/shop).

Referring to FIGS. 1 and 5, the distal frame FA2 includes a plurality ofupper loading members LMu, each of which can be disposed along a lengthof the main beams FA2 a and secured to a desired position in relation tothe SM, to the cross beams FA2 c using fasteners. The upper loadingmembers LMu can be mounted to the cross beams FA2 c, as illustrated inFIGS. 1 and 5. Adding more cross beams FA2 c further increases therigidity of the distal frame FA2.

Referring to FIGS. 1-2, the base FA1 can be made from an aluminum orsteel plates to rigidly mount the distal frame FA2, the movable supportFA3, guide rods FA4, and the driving mechanism DM. In this respect, thebase FA1 can be made from rigid material and construction that cansupport the movable support FA3 and the distal frame FA2, as well as thedriving mechanism DM. The frame assembly FA also includes at least apair of rigid guide rods or rails FA4 that space and maintains thedistal frame FA2 spaced from the base FA1 at a desired distance, whichcan be adjusted, for example by replacing the guide rods havingdifferent lengths. Moreover, each guide rod FA4 can include threadedends that can be bolted securely against the base FA1 and the distalframe FA2 and to provide a sufficient tension therebetween, as well asallowing fine height adjustments. Although the illustrated embodiment isshown with just a single pair of guide rods FA4, additional pairs ofguide rods can be included, particularly when the width of the movablesupport FA3 becomes larger, such as when the movable support FA3includes multiple lower bending members FA3 a, such as schematicallyillustrated in FIG. 6. In this respect, each of the main beams FA2 a caninclude at least a pair of through holes FA2 e spaced along the lengthdirection thereof each for receiving a guide rod FA4. See FIGS. 3 and 5.Similarly, the movable support FA3 also includes at least one pair ofthrough holes FA3 a each for receiving a guide rod. See FIGS. 1-3, 5,and 6.

Moreover, the guide rods FA4 further allow the movable support to slidetherealong as the movable support moves between the base and the upperframe. At least one lower loading member LMl is secured to the movablesupport. FIG. 1 shows a single lower loading member LMl and two upperloading members LMu to provide a three-point bending deflection system,while FIG. 6 shows two lower loading members LMl disposed at the movablesupport and two upper loading members to provide a four-point bendingdeflection system. This embodiment also can be used with just a singlelower loading member LMl (illustrated in phantom).

The movable support FA3, as well as each of the upper and lower loadingmembers LMu, LMl, can be made from a block of plastic material, such ashigh density polyethylene, aluminum, steel, wood, or a compositematerial, such as carbon fiber. Each loading member LMu, LMl can becontoured to the match the shape of the SM for better support, and has awidth that allows the SM to bend along multiple points. For example, thewidth of the loading members can range from 1 cm-15 cm, depending on thelength of the SM.

The driving mechanism DM can be any conventional commercially availablemechanical devices that can move the movable support toward or away fromthe distal frame FA2 (or away or toward the base FA1). For example, thedriving mechanism DM can be a linear jack screw, such as a worm gearscrew jack (Classic, IMA Series) available fromhttps://www.seshagirigears.com or from McMASTER-CARR(https://www.mcmaster.com/screw-jacks). Rotating a shaft, such as usinga motor or hand crank, translates a rotary motion of the crank into alinear motion, namely pushing or pulling motion.

Referring to FIGS. 1-2, a jack screw type driving mechanism can includesa main housing DM1, a driving shaft DM2, a hand crank DM3, a drivenshaft DM4, a driving-worm gear (not illustrated), a driven-worm gear(not illustrated), and a support platform DM5. The driven shaft DM4 isrotatably supported and typically includes an external acme threadingalong the substantial length thereof. The driving-worm gear is coaxiallyarranged with the driving shaft DM2, which is rotatably mounted to themain housing DM1, so that it can only rotate. That is, the driving shaftis held so that it cannot move along the axial direction of the drivingshaft. The driving-worm gear can have external worm gear teeth, whichcan be integrally formed with or mounted to the driving shaft DM2. Thehand crank DM3 is mounted to the driving shaft DM2 so that rotating thehand crank rotates the driving shaft DM2, which rotates the driving-wormgear in the direction of the rotation of the hand crank.

The driven-worm gear is coaxially arranged with the driven shaft DM4 andcan include external worm gear teeth that mesh with the external gearteeth of the driving-worm gear. The driven worm gear further includes aninternal acme threading that meshes with the external acme threading ofthe driven shaft DM4. The driven-worm gear is rotatably mounted to themain housing but is prevented from moving in the axial direction of thedriven shaft DM4. Thus, rotating the driven-worm gear via the drivingshaft DM2 (by rotating the crank DM3) rotates the driven shaft DM4. Thiscauses the driven shaft DM4 to move axially, either away from or towardthe main housing, depending on the rotation direction of the crank.Instead of the crank, a motor can be used to drive the driven shaftusing a controller or a computer.

The support platform DM5 is rotatably mounted to the distal end of thedriven shaft DM4 to enable the driven shaft DM4 to rotate relative tothe support platform DM5, such as using a bearing. The support platformDM5 supports the movable support FA3 so that raising the supportplatform DM5 raises the movable support toward the distal frame FA2.

The upper loading members LMu and the at least one lower loading memberLMl, which are configured to provide multiple bending points, aredisposed facing each other to apply an opposing force to the SM from theopposing sides. With the SM sandwiched between the upper loading membersLMu and the at least one lower loading member LMl, rotating the crankDM2 to rotate the driven shaft LM4 and elevate the support platform LM5toward the distal frame FA2, moves the movable support FA3 in the samedirection to apply a load to the SM.

The first sensor S1 can be a conventional load cell disposed between thedriving mechanism DM and the movable support FA3 for measuring the forceapplied by the driving mechanism DM to the SM. For example, the sensorS1 can be used include a load cell, such as Honeywell Model 31(https://sensing.honeywell.com/test-measurement-products/miniature-stainless-steel-load-cells/model-31)and Model 41(https://sensing.honeywell.com/test-measurement-products/stainless-steel-low-profile-load-cells/model-41). Specifically, the load cellcan be disposed underneath the movable support FA3, between the movablesupport FA3 and support platform DM5 so that it can measure the amountof force the driving mechanism DM is applying to the SM.

The second sensors S2, on the other hand, each are mounted to the distalframe FA2 and positioned along the length thereof at desired positions,and substantially aligned centrally with the sensor S3, for measuring adeflection around the SM. Each of the sensors S2 include a contact padS2 a, which is adjustably displaceable longitudinally (e.g.,perpendicular to the longitudinal direction of the distal frame FA2) topermit each pad to be in contact with the SM during measurements. Thethird sensor S3, on the other hand, is a non-contact type, and uses aradio frequency antenna RFA disposed spaced from the SM and mounted tothe distal frame FA2 to measure the deflection of the SM without usingany strain sensing device attached to the SM and/or the materialsurrounding the SM. There is no contact between the SM and the antennaRFA.

As noted earlier, the present development uses two distinct types ofdisplacement sensors S2, S3 that can be mounted to the distal frame heldstationary in relation to the base FA1. In the illustrated embodiment,the first type can be three linear displacement sensors (hereafterdisplacement sensor), such as DVRTs (Differential Variable ReluctanceTransducer) or LVDT (Linear Variable Differential Transducer),positioned along the length of the distal frame to function as lineardisplacement sensors to measure the displacement of the material (e.g.,skin) surrounding the SM. As illustrated, two linear displacementsensors S2 are located adjacent to two upper loading members LMu whilethe third linear sensor S2 is located at the midpoint between the twoupper loading members LMu. The two outer displacement sensors LMu canmeasure the baseline displacement due to compliance in the material(e.g., soft tissue) surrounding the loaded SM. The centrally locateddisplacement sensor S2 measures the total displacement due to thematerial (e.g., soft tissue) compliance when loaded. Subtracting theaverage displacement of the two outer displacement sensors S2 from thedisplacement of the central displacement sensor S2 results in thedeflection of the loaded SM due to bending.

The displacement sensors S2, as mentioned can be DVRTs or LVDT, whichare commercially available, for example, from MicroStrain/LORD Sensing(https://www.microstrain.com/displacement/nodes) or Omega Engineering(https://www.omega.com/subsection/displacement-proximity-transducers-all.html).

Referring to FIG. 7, the second type of deflection sensor is a radiofrequency antenna RFA, which can be powered by a network analyzer 26that includes an A/D converter. The network analyzer measures the S11parameter (return loss) of the antenna RFA. The resonant frequency ofthe antenna is determined as the frequency location of the local minimaof the S11 parameter dB response. The antenna's resonant frequency issensitive to the location of objects in the near field, and isespecially sensitive to metallic objects. Therefore, the antenna RFA canfunction as a proximity sensor and can measure deflections of the SM,such as extremity together with the implanted hardware, which istypically metallic.

The deflection measured from the antenna RFA is advantageous because itdoes not contact the SM and measures the deflection of the construct asa whole instead of relying on skin deflection of an extremity forexample. The displacement sensors S2 are advantageous because they canprovide displacement measurements at multiple locations along the lengthof the loaded extremity, allowing for accounting displacements due tosoft tissue compliance. The stiffness of the target construct can beobtained from the slope of the deflection curve versus the applied loadcurve.

Specifically, referring to FIGS. 3-5, the antenna RFA can have a dipoleconfiguration, using a pair of coiled coaxial cables CC1, CC2 (althoughnon-coiled type antennas can be used). But only one of the two cables,such as cable CC1, is connected to the network analyzer to obtain theS11 parameter. Using the center conductor W1, the S11 parameter, whichcan be measured with the cable CC1 connected to port 1, represents theratio of the power sourced at port 1 that is returned back to port 1,also known as the “return loss.” At the antenna's resonant frequency,the S11 parameter reaches a local minima. The outer shield of the othercable CC2 of the antenna RFA can be grounded to the outer shield of thecable CC1 that is grounded to the port 1.

For example, referring to FIG. 5, the outer shields of both coaxialcables, at one (first and third) ends thereof, can be electricallyconnected to each other at a coaxial connector CC1 a that allowsconnection of an extension coaxial cable that connects to port 1 of thenetwork analyzer. For example, this can be achieved by connecting theshield CC2 s (FIG. 5) of the cable CC2 to the connector CC1 a providedat the cable CC1. Alternatively, as illustrated in FIG. 3, the cable CC2also can include another coaxial connector CC2 a at the one (third) endand secure the two metal connectors so they make good contact and permitelectrical connection. Alternatively, connectors CC1 a can use a Y-typeconnector (not shown) to connect both the connectors CC1 a and CC2 a toground the shields to each other. Although FIG. 5 illustrates the bottomview of FIG. 3, the embodiment of FIG. 5 includes a terminating cap CC2c instead of the connector CC2 a.

It should be noted that the center conductor W2 of the cable CC2 isisolated from the coaxial connector CC1 a (or the Y-type connector). Atthe other (second and fourth) ends of the cables CC1, CC2, the outerjackets and the shields, and any foil shields are stripped off to exposethe dielectric insulators CC1 b, CC2 b. Moreover, referring to FIGS.3-5, part of each exposed dielectric insulator is stripped off to exposethe respective center conductors W1, W2.

Still referring to FIGS. 3-5, a mount M is installed on the distal frameFA2 using brackets FA2 d and fasteners. The mount M maintains theantenna RFA in a static position in relation to the distal frame FA2.Specifically, the mount M includes a pair of spaced plates M1, M2,preferably non-metallic, that are spaced apart. The pair of plates M1,M2 include a plurality of through holes, through which the cables CC1,CC2 are inserted through to provide a coiled configuration for each ofthe cables CC1, CC2.

Referring to FIG. 4, which provides an enlarged schematic view of the(second and fourth) ends of the cables CC1, CC2, the exposed centerconductors W1, W2 are held substantially parallel to each other. Thiscan be achieved by securing the ends of the exposed dielectricinsulators CC1 b, CC2 b in the pair of spaced through holes in the plateM2, such as by hot gluing them in place in the through holes, while theyare aligned substantially parallel. The exposed center conductors alsocan be bent to position them substantially parallel to each other. Thisallows the center conductors W1, W2 of the two coaxial cables CC1, CC2to interact with each other via their electric fields. These centerconductors W1, W2 are spaced from each other at the second and fourthends, but otherwise just terminate without connecting to anything. Onlythe center conductor W1 of the cable CC1 at the first end is used toconnect to the network analyzer 26. The length of the exposed centerconductors W1, W2 can be in the range 0.5 cm-20 cm. In the presentembodiment, the ideal length of the exposed center conductors W1, W2 isaround 4 cm.

The cable CC2 is merely a dud as, while the shields of both cables aregrounded, the center conductor of the cable CC2 is not connected to thenetwork analyzer or anything else. Although the cable CC2 is notdirectly used in signal analysis, it can help to eliminate noisecompared to a monopole antenna. For either antenna configuration, the SMaffects the frequency at which the local maxima or minima occurs whenthe SM is loaded.

Specifically, a metal plate or rod associated with the SM (e.g., afractured tibia fixed with an intramedullary nail) corresponding to SMcan be interrogated from the side of the coiled cables. Certainlocations along the length of the coaxial cable has the greatest shiftin signal. These locations depend on the resonant frequency harmonicthat is measured. The coiled shape increases the signal strength byaligning the locations along the cable length where the signal isstrongest. The antenna parameters can be optimized by adjusting theantenna length, spacing between coils, and resonant frequency harmonic,while interrogating a reference stainless steel bar at known distancesfrom the antenna. Increasing the spacing between the coils increases thesignal strength but also increases noise. Higher frequency harmonicsalso result in a stronger signal but greater noise.

Upon directing and emitting an alternating magnetic or electromagneticfield through a pre-determined frequency sweep using a source integratedin the network analyzer (or an external source if the network analyzeris not provided with the source) toward the SM, the SM interacts withthe applied electromagnetic field through near field effects. The SMbecomes electromagnetically coupled to the antenna RFA. The distancebetween the SM and the antenna can be represented by characteristics ofthe electromagnetic field coupling between the antenna and the SM. Asthe distance between the antenna and the SM changes, the characteristicsof the electromagnetic field coupling between the antenna and the SMshifts because the fundamental coupling between the antenna and the SMbecomes altered. For example, inducing deflection in the SM by applyinga load using the driving mechanism DM changes the distance between theantenna and the SM, while the antenna RFA remains fixed in spacerelative to the distal frame FA2, resulting in an alteration in theresonance frequency due to changes in the electromagnetic couplingbetween the antenna RFA and the SM.

The S-parameters of the antenna can be obtained by the connected networkanalyzer 26 or the connected analyzer/computer 28. The analyzer/computer28 can determine the resonance frequency, as well as the S-parametermagnitude of the antenna if coupled to the SM via the A/D converter,without the need for any strain sensor directly attached to the SM. If aseparate source, which can be an inductor or other conventionalapparatuses for applying electromagnetic fields in the radio frequencyspectrum, the network analyzer can just be an A/D converter, and theanalyzer/computer 28 can execute the functions of the network analyzer26 using software.

The antenna RFA is spaced from the SM so that it does not touch anysurface of the SM or the SM and/or surrounding material during theoperational conditions. The electromagnetic field surrounding theantenna RFA is affected by objects in the near field range due to theconductive and/or dielectric properties of the SM. A conductive materialhas an eddy current induced, which in turn causes the material to act asan antenna itself, thus altering the electromagnetic field. Anon-conductive dielectric material can also alter the electromagneticfield via the electromagnetic polarization of the material. Therefore,the SM can be any material that is conductive and/or has a relativepermittivity (i.e., dielectric constant) that is different than therelative permittivity of the surrounding medium (e.g., air).

The components of a monitoring system 20 can include the networkanalyzer 26, such as a commercially-available Tektronix TTR503A NetworkAnalyzer and Rohde & Schwarz ZVB4, which can apply electromagneticfields in the radio frequency spectrum, the antenna interface, whichincludes the antenna RFA, including any wire extension extending fromthe antenna wires, and the analyzer 28, which can be a computer thatreads and analyzes the data output from the network analyzer or storedover a period, or otherwise receives data that has been accumulated overthe period. The operating frequency range of the present monitoringsystem can be 10 MHz to 4 GHz, with the preferred range being 40 MHz to500 MHz for biomedical applications.

FIG. 8 schematically illustrates the analyzer 28, which comprises acontroller or computer that can be programmed to analyze the shifts inthe characteristics of the electromagnetic field coupling between theantenna RFA and the SM over an evaluation period or a predeterminednumber of measurements of the characteristics of the electromagneticfield coupling between the antenna and the SM obtained over apredetermined interval. The computer includes CPU (processor) 28A,memory 2(B), I/O (input/output) interface 28C. The I/O interface 28C caninclude a communication interface, such as Ethernet, for communicationto a network and Internet, a display interface for connecting to adisplay device 21, and typical interfaces, such as USB, for connectingperipheral devices, including a keyboard and a mouse, as well as thenetwork analyzer or any other device that can obtain the frequency sweepfrom the electrical signals obtained from the antenna RFA. The networkanalyzer 26 can be either a standalone apparatus, which can also beconnected to the computer via the I/O interface 28C, or a peripheraldevice that converts the electrical signals from the antenna RFA intodigital signals (e.g., ND converter) readable by the computer, and canbe connected to the computer 28 via either the Ethernet, USB or serialport.

The computer 28 can determine the characteristics of the electromagneticfield coupling between the antenna and the SM from the electrical signaldata obtained by the network analyzer 26 across a pre-determinedfrequency range. The functions of the network analyzer are well knownand are commercially available either as a standalone unit or softwareoperated unit using an A/D converter, such as a commercially availableTektronix TTR503A and Rohde & Schwarz ZVB4 network analyzers.Alternatively, the computer 28 can analyze the stored data of thecharacteristics of the electromagnetic field coupling between theantenna and the SM determined and read over an evaluation period or apredetermined number of times read over a predetermined interval by thenetwork analyzer 26. The storage device can be a memory drive within thecomputer itself, flash memory, network drive, or remote databaseconnected over the Internet.

The memory 28B communicates with the CPU 28A via a bus. The memory 28Bcan include a ROM 2861 and a RAM 12862. The memory 28B also can beconfigured as a non-volatile computer storage medium, such as a flashmemory, instead of the RAM and the ROM. The computer 28 can also includea removable memory (e.g., flash card) connected via the I/O interfaceusing, for example, USB or any other conventional memory card interface,and conventional hard disk 28D. The memory 28B and hard disk 28D aresome embodiments of a non-transitory machine-readable medium that canstore instructions, which, when executed by the processor, that performvarious operations. These operations include, but are not limited to,controlling/operating the source connected to the I/O interface 28C,controlling/operating the network analyzer connected to the I/Ointerface 28C, determining the characteristics of the electromagneticfield coupling between the antenna RFA and the SM, determining the shiftin characteristics of the electromagnetic field coupling between theantenna and the SM based on the electrical signals received from theantenna RFA in response to an electromagnetic field applied at differenttimes over the evaluation period, and determining the temporal change inthe displacement of the SM per the applied load, based on the shift incharacteristics of the electromagnetic field coupling between theantenna and the SM.

FIG. 9 illustrates an operational flow of the monitoring system that canmonitor the changes in the SM based on the antenna RFA. After the SM hasbeen secured to the loading device, at S100, the network analyzer 26 (orthe external source) is controlled to generate, emit, or direct anelectromagnetic field over a pre-defined frequency bandwidth towards theSM using the antenna RFA. The SM interacts with the emittedelectromagnetic field when subject to an alternating magnetic orelectromagnetic field. In S101, the network analyzer 26 outputs a(first) signal to the antenna RFA, which causes the antenna RFA tooutput an (second) electrical signal based on the coupledelectromagnetic field. The output electrical signal received from theantenna RFA is input to the network analyzer 26. After either thenetworks analyzer 26 or the controller determines the initialcharacteristics of the electromagnetic field coupling between theantenna RFA and the SM, which represents these parameters at the initialor start period, they can be stored in any of analyzer/computer 28,remote database, or local/portable storage device.

At S102, after determining the initial characteristics of theelectromagnetic field coupling between the antenna RFA and the SM, atimer or a counter is reset (i.e., evaluation starting point at whichthe initial characteristics of the electromagnetic field couplingbetween the antenna RFA and the SM has been set as a reference point).For accurate positioning of the SM after setting the reference, theinitially read (reference) parameters can be used as a reference toaccurately position the SM for taking subsequent multiple measurements.The timer/counter can be set using the controller or a standalone timer.Alternatively, the technician monitoring the target area can keep acalendar or manually keep track of the time and date as to when thecharacteristics of the electromagnetic field coupling between theantenna RFA and the SM are read in relation to the applied load. At apredesigned or desired time interval after the initial characteristicsof the electromagnetic field coupling between the antenna RFA and the SMhave been determined, process/step S103 essentially repeats S100, andprocess/step S104 (corresponding to S204 in FIG. 10) repeatsprocess/step S101 to determine the current characteristics of theelectromagnetic field coupling between the antenna RFA and the SM underload applied by the driving mechanism DM.

At S105, the analyzer or computer 28 can analyze the previouslydetermined characteristics of the electromagnetic field coupling betweenthe antenna RFA and the SM and the currently determined characteristicsof the electromagnetic field coupling between the antenna RFA and the SMand determine the shift of the characteristics of the electromagneticfield coupling between the antenna RFA and the SM by comparing thedetermined characteristics of the electromagnetic field coupling betweenthe antenna RFA and the SM over different times. The shift correspondsto the change in the distance between the antenna RFA and SM during theloading. Since the data is stored, the shift in the characteristics ofthe electromagnetic field coupling between the antenna RFA and the SMcan be determined after a predetermined number of characteristics of theelectromagnetic field coupling between the antenna RFA and the SM over adesired evaluation period has been read, or after the desired evaluationperiod has lapsed (where a desired total number of characteristics ofthe electromagnetic field coupling between the antenna RFA and the SMfor the desired evaluation period has been determined) (see S107). Inthis respect, the slope can be calculated from the change in thedistance versus the applied loads (curve).

At S106 the analyzer 28 determines whether the evaluation period haslapsed or the desired total number of characteristics of theelectromagnetic field coupling between the antenna RFA and the SM hasbeen made after determining each of the characteristics of theelectromagnetic field coupling between the antenna and the SM other thanthe determination of the initial characteristics of the electromagneticfield coupling between the antenna RFA and the SM. If the negative (NOin S106), after the preset interval, which corresponds to the durationor the interval between evaluations over the evaluation period, haslapsed (YES) at S108, processes/steps S103-S105 are repeated untilaffirmative in process/step S106 (YES). If affirmative (YES in S106),the analyzer 28 ends the evaluation since the evaluation period haslapsed. The temporal changes in relative displacement of the SM aredetermined based on the determined shift over the evaluation period. Theactual temporal changes in the displacement of the SM can be determinedby implementing an a priori deformation-electrical parameter ordisplacement-electrical parameter calibration of the hardware. Data froman electrical parameter-deformation or electrical parameter-displacementcalibration, performed in advance, can be stored in memory 28Baccessible by the analyzer 28.

For example, the electrical parameter signal, such as resonantfrequency, can be calibrated to correspond to the distance of theantenna RFA from surface of the SM. This can be done by measuring theactual distance from the SM in relation to the resonant frequency. Theresonant frequency measurement can then be used to determine therelationship between distance and resonant frequency. Because therelationship is known between the resonant frequency and the distance,the relationship can be determined between the resonant frequency andthe distance. The resonant frequency measurement can therefore becalibrated to give a direct measure of the distance for that particularSM, environment, and antenna setup, namely making the antenna functionas a displacement sensor.

The shift in characteristics of the electromagnetic field couplingbetween the antenna RFA and the SM can be used rather than the absolutevalues of the determined characteristics of the electromagnetic fieldcoupling between the antenna RFA and the SM in determining the temporalrelative changes in displacement of the SM. Based on the temporalchanges in relative displacement of the SM, changes in the target areaor biological subject can be determined. For instance, for a fracturefixation plate implanted in a person, these changes can be monitored foruse in the diagnosis and the prognosis for the healing of a fracturedbone for instance, or a condition of the SM.

When applying the present methodology to fracture healing, the objectiveis to determine the level of healing that has occurred by testing themechanical stability of the bone-implant construct. As the healingprogresses, the stiffness increases. As the stiffness increases, thedeflection of the construct relative to the applied load decreases, andthe signal from the antenna, such as resonant frequency shift, alsodecreases because it is a measure of the construct deflection.Calculating the shift in resonant frequency relative to the load appliedto the bone-implant construct provides a measure of the constructstiffness and thus the shift in the mechanical stability. Therefore, theslope of the resonant frequency versus the applied load curve can becalculated. This slope is a good indicator of the stiffness. Bydetermining this slope over time and comparing it to initialmeasurements, one can determine how the fracture is healing over thattime frame.

This methodology is particularly useful for monitoring the relative loadon the implant (due to the stiffness of the bone) at predetermined timepoints, such as every two weeks, throughout the healing of the fractureto monitor or predict the healing progress. As a fracture heals, the newtissue that grows progressively stabilizes the fracture, and thereforeincreases the relative load borne by the bone and decreases the relativeload borne by the implant (orthopedic plate or intramedullary nail). Asthe load on the plate decreases, the deformation of the implantdecreases proportionally, and the characteristics change accordingly,reflecting the stiffness of the bone. Calculating the shift in thesignal, such as resonant frequency, relative to the load applied to theimplant-bone construct reflects a measure of the relative load on theimplant, which corresponds to the amount of deflection or change in theamount of flexing. Therefore, the signal from the antenna can be plottedagainst the load applied to SM and the change in the deflection amountor the amount of distance moved by the load. The slope of the resultingcurve can represent the stability of the SM and the level of healing. Ifthe fracture is not healing properly, the load on the plate changesslowly or does not change over time, which means the amount ofdeflection does not change. By taking temporal measurements, a physiciancan monitor the healing progress by determining the change in thedeflection amount relative to the initial measurement. The measurementcan therefore provide the physician with an early indicator whether thefracture is not healing normally and may need further treatment.

While the data from the antenna RFA is being collected, the data fromthe load sensor S1 and the displacement sensors S2 can be collectedconcurrently or serially from a signal conditioner(s) 24, which isconnected to the ND converter (or network analyzer) 26 that converts theanalog signals output by these sensors to digital signals via ports 3-6.Some examples of signal conditioners include, for the DVRT displacementsensors, LORD MicroStrain, model DEMOD-DC(https://www.microstrain.com/displacement/DEMOD-DC) or model DEMOD-DVRT(https://www.microstrain.com/displacement/DEMOD-DVRT-2).

The load force data output by the load sensor S1 and the deflectionsamount output by the displacement sensors S3 all can be tracked, whilecalculating the shift in signal obtained via the antenna RFA via theport 1 of the network analyzer 26. Specifically, referring to FIG. 10,at process/step S202, after the reset and the timer has been initiatedin S102 in FIG. 9, the driving mechanism DM applies a desired load tostart the evaluation. In process/step 204, which corresponds to S104timewise, the network analyzer 26 receives the electrical signals fromthe sensors S1, S2, and S3. In process/step S206, which corresponds toS104 timewise, the network analyzer 26 receives the applied loadcorresponding to the signal from the load sensor S1 and the displacementcorresponding to the signal from each of the displacement sensors S2. Inprocess/step S208, the analyzer 28 calculates the slope from thedisplacement data (stored) from each of the displacement sensors S2versus an applied load curve (multiple applied loads). In process/stepS210, which corresponds to S106, the analyzer 28 determines whether theevaluation period or the desired total number of characteristics of theelectromagnetic field coupling between the antenna RFA and the SM haslapsed. Process/step S212 corresponds to S108. If the evaluation periodhas not lapsed, S202-S210 are repeated until affirmative YES in S210(i.e., the evaluation period ends). If affirmative in S210, the analyzer28 ends the evaluation.

In the present development, testing was conducted on a bone, namely afractured tibia fixed with an intramedullary nail or rod, which ismetal. But it should be noted that the SM to be tested is not limited tobone medium, but can be applied to any structural material.

Testing I: Healthy Human Tibia

The loading device was tested on a healthy tibia of a volunteer. Theloading frame was set in a three-point bending configuration asillustrated in FIG. 1 with the upper two loading members LMu 35cm apart,and the lower loading member LMl at the middle thereof. The loading wasapplied using the driving mechanism DM for five cycles from 0 to 100 N.Separate tests were conducted using the antenna RFA and the displacementsensors S2. The antenna test showed a consistent linear response of theresonant frequency relative to the applied load, as shown in FIG. 11.The mean slope from five cycles of loading was −0.082 MHz/N, and thestandard deviation was 0.0027 MHz/N, which is 3% of the mean.

The displacement sensors S2 were tested in a series of experiments toevaluate the corrected displacement measurement used to account forcompliance of the soft tissue. The corrected displacement was defined asthe center displacement minus the average of the two outsidedisplacements. Experiments were conducted using three methods to achievevarious degrees of compliance. First, the loading device was applied inits normal state (A). Second, the loading was applied in a padding state(B) where foam padding was added to the loading points to increase thecompliance. Third, the loading device was applied in a strap state (C)where the padding was removed, and the loading points were strapped tothe tested leg and tightened to decrease the compliance. The leg wasloaded for five cycles, and each test was conducted for threerepetitions. Slopes of the displacement/load curves were calculated forboth the raw center displacement sensor's displacement and the correcteddisplacement.

Referring to FIG. 12, as expected, the center displacement was highlyvariable between the three compliance groups (A)-(C) because the centerdisplacement includes the displacement due to compliance and thedeflection due to bending. The displacement due to compliance should bevariable between the three groups, but the bending deflection wastheoretically the same. Because the corrected displacement should be ameasure of only the bending deflection, it should be the same betweenthe three groups. The results showed that the corrected displacement wasless than the center displacement and had lower standard deviations thanthe center displacement. There were also smaller differences between thethree groups for the corrected displacement, compared to the centerdisplacement. The range of the three groups was 30% of the mean, and 83%of the mean for the corrected and center displacements, respectively.The corrected displacement therefore is an improvement over the rawcenter displacement for lowering the variability and accounting forchanges in compliance.

Additional experiments used the corrected displacement sensorsdisplacement method to test three loading configurations, where the twoupper loading members were spaced 35 cm, 23 cm, and 18 cm apart, whileusing a single lower loading member located centrally of the two upperloading members. Changing the spacing changes the applied bending momentrelative to the applied load, while changing the deflection due tobending. Experimental results were compared to analytical solutions ofbeam defection (δ) due to three-point bending, which were calculatedusing the equation:

${\delta = \frac{{Pl}^{3}}{48{EI}}},$

where P is the applied load, l is the length between the upper loadingpoints, E=20 GPa is the elastic modulus of cortical bone according toYoung's Modulus of Trabecular and Cortical Bone Material: Ultrasonic andMicrotensile Measurements, Jae Young Rho, Richard B. Ashman, and CharlesH. Turner, 1993 Journal of Biomechanics, Volume 26, Issue 2: pp.111-119, and I=1.5 E⁻⁸ m⁴ is the area moment of inertia of a human tibiaaccording to The Human Tibia A Simplified Method of RadiographicAnalysis of its Cross-Section, with Anthropometric Correlations, Ira D.Stein and Gerald Granik, 1979 Annals of Biomedical Engineering, Volume7, pp. 103-116.

Analytical solutions were also calculated for titanium rod of 10 mm and12 mm diameters representing an unhealed (post-operational) fracturefixed with an intramedullary rod to compare the testing results on anintact tibia to the expected results for a fractured tibia. Theexperimental results showed relatively good agreement with theanalytical solutions in FIG. 13. Discrepancies were likely due to themoment of inertia and elastic modulus properties assumed in theanalytical solution, which can be highly variable. Large differenceswere seen between the analytical solutions for the intact tibia and thefractured tibia (intramedullary rod), indicating that differences in thebone deflection can be tracked over time as a fracture heals.

Testing II: Fracture Model of Sheep Tibia

The loading device was also tested on a cadaveric sheep tibia with animplanted intramedullary rod. The tibia was tested in eight conditionsto simulate various stages of fracture healing. The first test wasconducted with rod implanted in an intact bone. For tests two throughsix, progressively larger osteotomies were made to simulate a partiallyhealed transverse diaphyseal fracture. For test seven, the osteotomy wasextended entirely through the cross section of the bone to simulate anunhealed (post-operational) fracture. For test eight, a second completeosteotomy was made to simulate an unhealed segmental fracture.Radiographs were used to guide and confirm the osteotomies. See FIG. 14.

The loading device was configured in three-point bending deflection withthe two upper loading members spaced 18 cm apart and a single lowerloading member located centrally of the two upper loading members.Deflection data were collected from the antenna RFA and the displacementsensors S2 simultaneously. In each test, the bone was loaded from 30 Nto 130 N for five cycles, and slopes were calculated from the correcteddisplacement sensor displacement versus load curves and the antennaresonant frequency versus the same load curves.

Referring to FIGS. 15A-15B, results using the antenna data showed thatthe resonant frequency shift increased relative to the applied load witheach progressive osteotomy condition, indicating that the slope of theresonant frequency versus the load curve was able to track the decreasedstiffness of the bone-rod construct as it was progressivelydestabilized. The displacement sensor data showed a similar trend to theantenna data, where the slope of the displacement versus the same loadcurve was higher in tests six through eight than tests one through five.But the slopes did not increase in each of the first five tests testlike they did for the antenna data. Also, tests eight (segmentalfracture) had a lower displacement/load slope than test seven (singlefracture).

The greater variability in the displacement sensor data than the antennadata can be attributed to the displacement sensor measuring skindisplacement, whereas the antenna RFA measures the combined deflectionof the bone, soft tissue, and implanted metal rod, and is especiallysensitive to metal materials. The large amount of soft tissue around thesheep tibia relative to typical human tibias may have contributed to thevariability in the displacement sensor data. Also, the displacement/loadslope was likely lower for test eight than test seven because the centerdisplacement sensor S2 was located over the loose segment of thesegmental fracture and did not displace as much, despite the moredestabilized structure. But the antenna data did track the increaseddestabilization of test eight as an increased slope in the resonantfrequency versus the load curve. Overall, these results indicate thatthe present device is capable of tracking changes in construct stiffnessfor a fractured bone fixed with orthopedic hardware, especially usingthe antenna deflection measurement method.

In short, the above tests showed that the using both sensor types canmonitor fracture healing progress over time.

Given the present disclosure, one versed in the art would appreciatethat there may be other embodiments and modifications within the scopeand spirit of the present invention. Accordingly, all modificationsattainable by one versed in the art from the present disclosure withinthe scope and spirit of the present invention are to be included asfurther embodiments of the present invention. The scope of the presentinvention accordingly is to be defined as set forth in the appendedclaims.

What is claimed is:
 1. A loading device for measuring stiffness of astructural member (SM), the loading device comprising: a frame assemblyincluding: a base restable on a support; a distal frame held spaced fromthe base at a predetermined distance; a movable support movable betweenthe base and the distal frame; a plurality of first loading membersdisposed spaced along a length of the distal frame and configured tosupport the SM; at least one second loading member disposed on themovable support and configured to support the SM; a driving mechanismthat moves the movable support to apply a load to the SM and createmultiple bending points with respect to the plurality of first loadingmembers and at least one second loading member; a first sensor thatmeasures a load applied by the driving mechanism to the movable support;a plurality of second sensors that measure a first deflection of the SMundergoing loading; a third sensor that measures a second deflection ofthe SM undergoing loading without using any strain sensing deviceattached to the SM or contacting the SM, wherein the stiffness of the SMis determinable from: a first slope of a first deflection curve obtainedfrom the plurality of second sensors versus an applied load curveobtained from the first sensor; and a second slope of a seconddeflection curve obtained from the third sensor versus the applied loadcurve.
 2. The loading device according to claim 1, wherein: the firstsensor is disposed between the driving mechanism and the movable frame;each of the plurality of second sensors is adjustably positionable alongthe length the distal frame; the third sensor comprises an antennadisposed spaced from the SM, secured to the distal frame, and configuredto: induce, using a first electrical signal, a magnetic orelectromagnetic field in the vicinity of the SM to create a coupling ofthe magnetic or electromagnetic field between the antenna and the SM,where characteristics of the magnetic or electromagnetic field couplingbetween the antenna and the SM are associated with a distance betweenthe SM and the antenna; and output a second electrical signalrepresenting the magnetic or electromagnetic field coupling between theantenna and the SM, without using any strain sensing device directlyattached to the SM.
 3. The loading device according to claim 2, wherein:the plurality of first loading members and the at least one secondloading member are disposed facing each other, and the driving mechanismmoves the at least one second loading member toward the plurality offirst loading members to apply an opposing force to the SM from oppositesides.
 4. The loading device according to claim 3, wherein the pluralityof first loading members and the at least one second loading memberprovide a three-point bending or four-point bending configuration. 5.The loading device according to claim 4, wherein: the plurality of firstloading members include two first loading members, the at least secondloading member includes one second loading member, the plurality ofsecond sensors include three displacement sensors positioned along thelength of the distal frame, and two of the three displacement sensorsare disposed adjacent to the two first loading members, and the thirddisplacement sensor is located at the midpoint between the two firstloading members.
 6. The loading device according to claim 2, wherein theantenna is a dipole antenna that provides a S11 parameter, and functionsas a displacement sensor.
 7. The loading device according to claim 2,further comprising: a mount secured to the distal frame and holding theantenna, wherein the antenna comprises a first coaxial cable including afirst end and a second end, wherein the mount is configured to maintainthe first coaxial cable stationary, in relation to the distal frame, ina coil configuration between the first and second ends.
 8. The loadingdevice according to claim 7, wherein: the mount comprises a pair ofspaced plates secured to the distal frame, each of the pair of spacedplates includes a plurality of spaced through holes, through which thefirst coaxial cable is kept in the coil configuration.
 9. The loadingdevice according to claim 8, wherein: the antenna further includes asecond coaxial cable including a third end and a fourth end, wherein theplurality of spaced through holes also keep the second coaxial cable inthe coil configuration between the third and fourth ends.
 10. Theloading device according to claim 9, wherein: the first end of the firstcoaxial cable includes a connector for connecting to a signal processorso that only the signal from the first coaxial cable is used to obtainthe second deflection curve.
 11. The loading device according to claim10, wherein: the first coaxial cable includes a first shield and a firstcenter conductor, the second coaxial cable includes a second shield anda second center conductor, the first and second shields are electricallyconnected, the first center conductor at the second end and the secondcenter conductor at the third and fourth ends terminate without anyconnection, a predetermined length of the first center conductor isexposed at the second end and the predetermined length of the secondcenter conductor is exposed at the fourth end, and the exposed first andsecond conductors are maintained substantially parallel to each other.12. The loading device according to claim 1, wherein the SM is a bonestructure.
 13. A monitoring system for monitoring measuring stiffness ofa structural member (SM) over an evaluation period, the systemcomprising: a loading device comprising: a frame assembly including: abase restable on a support; a distal frame held spaced from base at apredetermined distance; a movable support movable between the base andthe distal frame; a plurality of first loading members disposed spacedalong a length of the distal frame and configured to support the SM; atleast one second loading member disposed on the movable support andconfigured to support the SM; a driving mechanism that moves the movablesupport to apply a load to the SM and create multiple bending pointswith respect to the plurality of first loading members and at least onesecond loading member; a load sensor that measures a load applied by thedriving mechanism to the movable support; a plurality of displacementsensors that measure a first deflection of the SM undergoing loading; anantenna that measures a second deflection of the SM undergoing loadingwithout using any strain sensing device attached to the SM or contactingthe SM, wherein the antenna is configured to: induce, using a firstelectrical signal, a magnetic or electromagnetic field in the vicinityof the SM to create a coupling of the magnetic or electromagnetic fieldbetween the antenna and the SM, where characteristics of the magnetic orelectromagnetic field coupling between the antenna and the SM areassociated with a distance between the SM and the antenna; and output asecond electrical signal representing the magnetic or electromagneticfield coupling between the antenna and the SM, without using any strainsensing device directly attached to the SM; a controller including amemory storing instructions and a processor configured to implementinstructions stored in the memory and execute a plurality of tasks; ahardware interface configured to receive the second electrical signalfrom the antenna, a third electrical signal from the load sensor, and afourth electrical signal from each of the plurality of displacementsensors, and convert each of the received second, third, and fourthsignals that are readable by the controller, wherein the plurality oftasks includes: a first determining task that receives the convertedsecond electrical signal from the hardware interface and determinescharacteristics of the magnetic or electromagnetic field couplingbetween the antenna and the SM; a repeating task that repeats the firstdetermining task to obtain a plurality of characteristics of themagnetic or electromagnetic field coupling between the antenna and theSM at a predetermined interval over the evaluation period; a seconddetermining task that determines a shift in characteristics of themagnetic or electromagnetic field coupling between the antenna and theSM after each occurrence of the first determining task determining thecharacteristics of the magnetic or electromagnetic field couplingbetween the antenna and the SM at the predetermined interval, orcollectively at the end of the evaluation period; and a thirddetermining task that determines a temporal change in relativedisplacement of the SM and determine the stiffness of the SM over theevaluation period, based on: a first slope of a first deflection curveobtained from the plurality of displacement sensors versus an appliedload curve obtained from the load sensor; and a second slope of a seconddeflection curve obtained from the antenna versus the applied loadcurve.
 14. The monitoring system according to claim 13, wherein thehardware interface is a network analyzer configured to: output the firstelectrical signal to the antenna to induce the magnetic orelectromagnetic field; receive the second electrical signal from theantenna; and also determine the characteristics of the magnetic orelectromagnetic field coupling between the antenna and the SM based onthe received second electrical signal.
 15. The monitoring systemaccording to claim 13, wherein the antenna is a dipole antenna thatprovides a S11 parameter, and functions as a displacement sensor. 16.The monitoring system according to claim 13, further comprising: a mountsecured to the distal frame and holding the antenna, wherein the antennacomprises a first coaxial cable including a first end and a second end,wherein the mount is configured to maintain the first coaxial cablestationary, in relation to the distal frame, in a coil configurationbetween the first and second ends.
 17. The monitoring system accordingto claim 16, wherein: the mount comprises a pair of spaced platessecured to the distal frame, each of the pair of spaced plates includesa plurality of spaced through holes, through which the first coaxialcable is kept in the coil configuration.
 18. The monitoring systemaccording to claim 17, wherein: the antenna further includes a secondcoaxial cable including a third end and a fourth end, wherein theplurality of spaced through holes also keep the second coaxial cable inthe coil configuration between the third and fourth ends.
 19. Themonitoring system according to claim 18, wherein: the first coaxialcable includes a first shield and a first center conductor, the secondcoaxial cable includes a second shield and a second center conductor,the first and second shields are electrically connected, the firstcenter conductor at the second end and the second center conductor atthe third and fourth ends terminate without any connection, apredetermined length of the first center conductor is exposed at thesecond end and the predetermined length of the second center conductoris exposed at the fourth end, and the exposed first and secondconductors are maintained substantially parallel to each other.
 20. Amethod of monitoring stiffness in a structural member (SM) as the SMundergoes a displacement under a load, the method comprising: disposingan antenna spaced from the SM so that the antenna does not contact theSM, wherein the antenna is configured to: induce, using a firstelectrical signal, a magnetic or electromagnetic field in the vicinityof the SM to create a coupling of the magnetic or electromagnetic fieldbetween the antenna and the SM, where characteristics of the magnetic orelectromagnetic field coupling between the antenna and the SM areassociated with a distance between the SM and the antenna; and output asecond electrical signal representing the magnetic or electromagneticfield coupling between the antenna and the SM, without using any strainsensing device directly attached to the SM; inducing the magnetic orelectromagnetic field in the vicinity of the SM and creating thecoupling of the magnetic or electromagnetic field between the antennaand the SM; applying a load to the SM to create multiple bending points;monitoring each load applied to the SM using a load sensor and storingeach load data in a storage device; measuring an amount of deflectionthe SM undergoes for each applied load using a plurality of displacementsensors each disposed adjacent to one of the multiple bending points,and storing deflection data in the storage device; obtaining the secondelectrical signal from the antenna each time a load is applied to the SMand determining a distance between the antenna and the SM for eachapplied load and storing determined distance information in the storagedevice; and determining a first slope of a first deflection curveobtained from the stored deflection data versus an applied load curveobtained from the stored applied load data; determining a second slopeof a second deflection curve obtained from the stored determinedcharacteristics versus the applied curve; and determining the stiffnessof the SM based on the first slope and the second slope.